To separate the gases in the air, their density property can be used.
Some gases present in the air have higher densities, other have lower densities. The gas like argon which has a lower density (=1.784g/L) is lighter and will be at the top.
On the other hand, a gas like carbon-di-oxide is heavier with high density (=1.98g/l). So, it will be at the bottom.
Oxygen is denser (=1.43g/l) than nitrogen (1.25g/l). So, nitrogen will be above oxygen in the atmosphere. Both oxygen and nitrogen will be above argon and carbon-di-oxide.
Nitrogen (top most)—à then oxygen—à then argon–à then carbon-di-oxide (With these varying densities, it is evident, that these can be separated from each other).
Note that centrifugal force can be used to separate a uniform mixture of gases.